CsI photocathode: New results on photon ageing.
Triloki* and B.K. Singh.High Energy Physics Laboratory, Physics Department, Banaras Hindu University, Varanasi221005, INDIA.
. * email: [email protected]
IntroductionIn the recent years
considerable progress
has been made in the development
of
gaseous electron multiplier (GEM) based photon detector consisting
a solid photocathode [1].
Cesium iodide (CsI) is known to
be the best photocathode as
photon converter. This
is because of their higher Quantum Efficiency (QE) in
Ultra Violet (UV), Xray UV
and
Xray energy ranges and their relatively high stability against
ambient air and gas environment
[2]. Photon detectors, which is
capable of single photon counting,
can reach dimension of
few square meter or more and can operated at high magnetic
field [3,4]. Such detectors
are employed in particle
and astroparticle
physics experiments for particle identification using Ring Imaging
CHerencov (RICH) technique. It
has been observed that the QE of CsI photocathode decreases
once exposed to photon
irradiation and/or by exposed to humid air [5] and hence it affects the detector performance.
The aim of present work is
to study
the degradation of CsI photocathode by UVphotons irradiation.
Experimental Details. The experimental
setup includes a high
vacuum spherical evaporation chamber. A high vacuum
(of the order of 107 Torr)
inside
the spherical chamber was created by the means of a Turbomolecular
pump. Before starting
the deposition process, Cesium Iodide (CsI) powder, of
5 N purity (Aldrich Chem Co.),
was melted
and out gassed from a molybdenum boat under the shutter. After proper out gassing and melting, CsI photocathode was deposited onto a polished aluminum
disc of 10 mm diameter.
The CsI evaporation rate was 12
nm/sec for 500 nm thick film.
After the film preparation,
nitrogen gas was flushed into
the chamber in order
to avoid humidity and for restoring the atmospheric pressure.
Immediately after the
chamber opening, CsI photocathode were inserted into a vacuum
desiccator and moved to the
QEmeasurement setup [6]. In
QEmeasurement setup, absolute QE
measurement, before
and after UVirradiation were done.
Fig: 1. Schematic diagram of
CsI evaporation apparatus.
Result and discussion. In order to study the effect of UV photon
irradiation, a 500 nm “as
evaporated” CsI photocathode was
illuminated with 160 nm monochromatic
photons. Fig. 2 shows
the variation of relative photocurrent as a function of
Proceedings of the International Symposium on Nuclear Physics
(2009) 652
accumulated charge. From this Fig,
one can observe that after
intense irradiation of
UV photons, a twentypercent loss (TPL) in relative photocurrent was observed after ~5 µC/mm2
for 500 nm thick CsI photocathode.
At the end of ageing measurement,
a 90% loss in
relative photocurrent was observed while
the accumulated charge was ~30 µC/mm2.
Fig: 2. Relative Photocurrent as a function of the accumulated charge during UV irradiation, under vacuum, of a 500nm CsI photocathode.
Fig: 3. Quantum Efficiency (QE) as a function of the wavelength of an “asdeposited” and
“UVirradiated” CsI photocathode.
In Fig. 3, the Quantum Efficiency (QE) of
CsI photocathode before and after
ageing test is displayed, which
shows decrease in QE
with increasing wavelength. It is observed that ageing induced by monochromatic UV photons leads to a decrease in QE at all wavelengths.
Conclusion.We have started new R & D work on the
photoemission properties of CsI
photocathode, for hadron blind detector
of PHENIX experiment. In this
work we have presented degradation
of CsI photocathode by
UV irradiation. We also presented result on
QE of “asdeposited” and
“UVirradiated” CsI photocathode for
various wavelengths.
Surface characterization of “asdeposited”
and
“aged” CsI photocathode will be done in future in order to
understand the ageing mechanism
due to photonirradiation.
Acknowledgment. The research work
partially supported by
Department of Science and Technology
(DST) and Council of Scientific and Industrial research (CSIR), govt. of India.
References
[1]
A. Breskin, et al., Nucl. Instr. Meth. A 371 (1996) 116 and references therein.
[2]
A. Breskin, et al., Nucl. Instr. Meth. A 387 (1997) 1 and references therein.
[3]
F. Piuz, Nucl. Instr. Meth. A 502 (2003) 76.[4]
E. Nappi, et al., Nucl. Instr. Meth. A 471
(2001) 18.[5]
B. K. Singh, et al., Nucl. Instr. Meth. A 581
(2008) 651.[6]
A. Valentini, et al., Nucl. Instr. Meth. A 482
(2002) 238.
Proceedings of the International Symposium on Nuclear Physics
(2009) 653
High Energy Physics Laboratory, Physics Department, Banaras
Hindu University, Varanasi-221005, INDIA.
